Sample line graph plot - what is the XY axis represent?
That plot you get is not an histogram. It is used for video analysis, it is called Waveform Monitor. And it is, as you correctly deduce, a plot of a spatial horizontal axis (pixel along the line), and a luminance vertical axis (brigthness for that pixel.

See the page as a reference: https://www.bhphotovideo.com/explora/video/tips-and-solutions/introduction-waveforms-scopes-and-exposure